1. Field of the Invention
The invention relates to belt tensioning devices, and in particular to mechanical spring-actuated belt tensioning devices for use with the endless drive belts of the drive systems for vehicle accessories. More particularly, the invention relates to a relatively simple and compact belt tensioner operated by a plurality of torsional springs, which achieves and maintains a predetermined tensioning force on the endless drive belt by engagement of the drive belt by an idler pulley or by pivotally moving one of the vehicle accessories in a belt tensioning direction.
2. Description of the Prior Art
There is the trend today in the automobile industry to operate the various vehicle accessories, such as the power steering pump, oil and air pumps, air conditioning and alternator, by a single endless belt driven by a pulley connected to the engine crankshaft. This system is referred to as a "serpentine" drive belt system. To ensure optimum operating efficiency for these various accessories, it is necessary that the drive belt be maintained at a predetermined tension to assure efficient performance of the accessories as well as satisfactory service life for the belt. Due to the relatively greater length of the single drive belt which replaces the heretofore plurality of smaller belts, there is a greater tendency for the belt to stretch which will affect the operating characteristics of the driven accessories. Therefore, it is desirable that a belt tensioning device be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension thereon regardless of the amount of belt stretch.
Numerous devices have been proposed and used to accomplish this purpose. One type of tensioner uses a bushing formed of an elastomeric material which is placed in compression by some mechanical means for continuously exerting a tensioning force on the belt. Examples of these constructions are shown in U.S. Pat. Nos. 3,975,965 and 4,144,772. These tensioner constructions which use an elastomeric material, have the disadvantages in that the high load rate which they exert on the belt results in the rapid loss of tensioning as the belt stretches, and this load rate limits the stroke of the belt-engaged idler pulley to a shorter distance than desired. Also, sudden acceleration and deceleration of the drive belt can cause a whipping action to occur which creates a time lag before full damping is achieved.
Numerous other types of belt tensioning devices use compression or tension springs for applying and maintaining the tensioning force on a belt-engaging idler pulley or chain-engaging sprocket. Some examples of these types of constructions are shown in U.S. Pat. Nos. 2,703,019, 2,893,255, 3,413,866, 3,483,763, 3,631,734, 3,768,324, 3,812,733 and 3,965,768. Other various spring-actuated devices use the biasing force of a tension or compression spring in combination with hydraulic actuated members for regulating the amount of tensioning force applied to the belt, depending on whether the engine is running or shut off. Examples of these combination hydraulic and spring-actuated tensioning devices are shown in U.S. Pat. Nos. 2,051,488, 3,142,193, and 4,077,272.
Due to the relatively large number of different vehicles, and the different drive belt and accessory arrangements therefor, it is difficult to provide a tensioner construction which can be adapted for use in nearly all of these various installations. Also, it is desirable that such a belt tensioner can exert the necessary tensioning force and have a compact, rugged and inexpensive construction to minimize both original equipment cost and installation cost, and which reduces or eliminates maintenance.
The above-listed examples of spring-actuated belt tensioners use a coil spring which is placed in compression or tension for applying the belt tensioning force. It is difficult to achieve high tensioning forces with springs which are placed in tension or compression in a relatively small compact unit due to the size of the springs required to develop relatively large tensioning forces.
One known belt tensioner construction, shown in U.S. Pat. No. 3,924,483, uses a torsional spring for pivotally moving one of the vehicle accessories to achieve the desired tensioning force. There is one other known belt tensioner construction which uses a pair of torsional coil springs for pivotally moving a lever and idler pulley into belt tensioning engagement which provides a relatively simple, economical and compact unit. This latter tensioning device uses two torsional coil springs, each being mounted on a respective side of the lever and engaged with the lever and housing for biasing the intervening lever in a belt tensioning direction. This tensioning device is believed generally satisfactory for many belt tensioning applications. However, certain applications require a greater tensioning force than that which can be obtained with this known torsional spring device without increasing the size of the unit to use larger torsional springs.
Some vehicle manufacturers desire a belt tensioning device which tensions the belt by pivotally moving one of the vehicle accessories, whereas other manufacturers prefer a tensioning device which is mounted as a complete unit on the engine and automatically tensions the drive belt by direct engagement therewith without pivotally mounting one of the vehicle accessories.
Therefore, the need has existed for a belt tensioner construction which is of a simple, rugged and compact design, which can be modified easily by adding additional coil torsional springs to achieve various belt tensioning loads without materially increasing the size of the device, and which permits flexibility in installation by pivotally mounting one of the vehicle accessories for tensioning the drive belt which is operatively engaged with said accessory, or else by directly engaging the belt by an idler pulley which is biased into belt tensioning engagement by the force of the torsional springs.
There is no known belt tensioning construction of which I am aware which imparts a generally constant predetermined tensioning force on an endless drive belt by moving an idler pulley into tensioning engagement with the belt or by pivotally moving one of the vehicle accessories, by the use of three or more coil torsional springs which are mounted in a telescopic relationship with respect to each other within a compact, rugged housing construction.